In the production of printed circuits, a protective film is applied to a printed circuit board in order to protect the electrical circuit and to prevent adhesion of solder material in undesired areas when electrical parts are being soldered onto the printed circuit board. The demand for light circuit boards while having a high density of circuits requires compositions having good adhesion properties, chemical stabilities and good electrical properties.
Conventional heat-curable and photopolymerizable compositions frequently comprise an epoxy compound and a photosensitive prepolymer. If such a composition is developed in an alkaline solution after drying and exposure, the unexposed parts of the photosensitive prepolymer are only poorly soluble. Moreover, the epoxy compound frequently reacts with the epoxide curing agent as early as during the drying step, which decreases the ability of development and leads to a poorly developable layer on the copper surface.
EP-A1-587 189 discloses a photocurable liquid solder resist ink composition comprising a photopolymerization initiator, a diluent and a photopolymerizable resin which is obtainable by reacting a novolak-type epoxy resin having 4 or more benzene nuclei with both (meth)acrylic acid and a chain-extension agent having 2 to more functional groups reactable with the hydroxyl groups and/or epoxy groups.
U.S. Pat. No. 6,077,879 discloses a radically polymerizable curable resin obtained by the reaction of an epoxy resin having 2 or more epoxy groups with a phenol compound having an alcoholic hydroxy group which is further reacted with unsaturated monobasic acid and an anhydride of a polybasic acid.
WO-A1-2004/081621 discloses a photosensitive resin composition for a colour filter wherein the photosensitive composition comprises a binder resin which is obtainable by a cyclopentadiene-novolak epoxy resin with an unsaturated carboxylic acid and an anhydride of a polybasic acid.
EP-A1-556 429 discloses an epoxy resin composition comprising a cyclopentadiene-novolak epoxy resin, at least a phenolic hydroxyl-containing compound and an epoxy-containing component which is different from the dicyclopentadiene-novolak epoxy resin.
JP-10-001596 discloses an acid group-containing epoxy vinyl ester resin obtainable by (i) a cyclopentadiene-novolak epoxy resin with (ii) an ethylenically unsaturated monobasic acid, followed by reacting the resulting epoxy vinyl ester resin with (iii) a polybasic anhydride to modify the OH of the resulting resins.
Solder resist compositions known in the prior art suffer from a poor ability to be developed and, additionally, they are too brittle, which leads to cracks under thermally changing conditions. In particular, for automotive applications solder masks are required which are stable under changing temperature conditions. Likewise, it is required that the solder mask has a high dielectric strength and an excellent moisture insulation resistance and does not suffer from electrochemical corrosion. Depending on the field of application the solder resist composition must additionally be stable under conditions applied during the immersion of nickel/gold as well as tin. Higher environmental standards in the automotive industry require lead-free solders which, however, have a higher melting point (up to 290° C.). Therefore, solder resist compositions are required which have an excellent solder resistance and high temperature resistance respectively.
Surprisingly, a photocurable composition has been found which overcomes the above-mentioned problems in the prior art. In particular a photocurable composition has been found which demonstrate an increased heat resistance and solder resistance respectively.